Since plastic spectacle lenses are easily scratched, it is customary practice to coat them with an abrasion-resistant hard coating. Generally, a lens is coated with a hard coating by applying a hard coating solution to the lens surfaces of the lens and then drying the applied hard coating solution into a hard layer.
Processes of applying a hard coating to a lens include a spin-coating process and a dipping process. The dipping process is mainly relied upon for its higher productivity.
According to the conventional dipping process, lenses are mounted on a lens holding jig which is designed to hold about 30 lenses simultaneously, and then the lenses are dipped, together with the lens holding jig, in a hard coating solution. After elapse of a predetermined period of time, the lenses are pulled up and then dried while being held by the lens holding jig.
However, lens holding jigs which are capable of holding many lenses have to be available in very many types depending on the lens diameter and the lens edge thickness. The cost of a facility for keeping such lens holding jigs is high and the management of those lens holding jigs is complex.
Since a lens holding jig needs to be selected depending on the diameter and thickness of lenses to be held, it is necessary to sort lenses into small classifications, and such a sorting process is tedious and time-consuming.
A lens holding jig for holding many lenses has a framework made up of many components. When the lens holding jig is dipped in a hard coating solution, the hard coating solution is applied to those framework components. Therefore, the applied hard coating solution is wasted, and the usability of the hard coating solution is low.
Particularly, recent years have seen many customized lenses manufactured in many types and small lots, and a reduced ratio by which a lens holding jig is filled with lenses. The above problems manifest themselves under these circumstances.
The present applicant has developed a single lens holding jig for holding one lens independently of each other, as disclosed in Japanese Patent Laid-Open No. 2003-71650. An example of the single lens holding jig for holding a lens with a thin edge is shown in FIGS. 9(a), 9(b), and 9(c) of the accompanying drawings. FIG. 9(a) is a front elevational view of the single lens holding jig, FIG. 9(b) is a view showing left and right side holders which are holding a lens, and FIG. 9(c) is a view showing a lower side holder.
The single lens holding jig, denoted by 500, has a lens holder 502 for supporting and holding an edge of a single upstanding lens L, and a carrier fixture 503 to which the lens holder 502 is attached. The carrier fixture 503 is engaged and suspended by a carrier jig, not shown. The lens holder 502 has a first arm 504, a second arm 505 and a third arm 506. The first arm 504 bends at a sharp angle from a helical spring and includes a first side holder 44 on its distal end which is biased for contact with an edge of the lens by the helical spring. The second arm 505 includes a second side holder 45 for contact with an edge of the lens which is opposite to the edge of the lens engaged by the first side holder 504. The third arm 506 includes a lower side holder 46 for contact with a lower edge of the lens between the edge of the lens engaged by the first side holder 44 and the edge of the lens engaged by the second side holder 45. As shown in FIG. 9(b), each of the first side holder 44 and the second side holder 45 is in the form of a metal wire bent into a V-shaped holder for holding the edge of the lens L. The lower side holder 46 is in the form of a flat plate having a V-shaped recess for supporting the lower edge of the lens L.
Since the lens holding jig 500 is a single single lens holding jig for holding a single lens, if lens holding jigs depending on the types of lenses are made available, then lenses of many types can be held by respective lens holding jigs 500, suspended by a single carrier jig, and dipped in a hard coating solution or the like. Therefore, the single lens holding jigs are suitable for use in the manufacture of lenses in many types and small lots. Since a single lens holding jig is suspended, the amount of hard coating solution that is applied to other than the lens is small, and the usability of the hard coating solution is high. In addition, inasmuch as the lens holder 502 is allowed by the helical spring to hold lenses of different diameters, the lens holding jig 500 of one type alone is capable of holding many types of lenses regardless of different lens diameters.
With the lens holding jig 500 shown in FIGS. 9(a) through 9(c), however, flow marks (thickness irregularities) 510 are produced by the hard coating solution, as shown in FIG. 10(a) of the accompanying drawings, on the surface of the lens L in those areas which are contacted by the first side holder 44 and the second side holder 45. If large thickness irregularities are produced, they tend to develop a defect in the appearance of the lens L because they are positioned in a edged lens shape that is placed in an eyeglass frame.
As shown in FIG. 10(b) of the accompanying drawings, if the positions where the lens L is pressed by the first side holder 44 and the second side holder 45 are lowered approximately to a horizontal line H passing through the center of the lens L, then thickness irregularities that are produced may be prevented from being positioned in the edged lens shape. However, because this lens holding arrangement holds the lens at three locations thereon, i.e., the left and right ends near the lens center and the lower lens end, the lens is not held stably and is liable to drop while a hard coating is being deposited on the lens, resulting in a reduction in the yield of lenses.
The present invention has been made under the above circumstances. It is a first object of the present invention to provide a lens holding jig which is capable of reducing, as much as possible, the generation of a lens appearance defect by side holders for holding left and right edges of a lens.
With the lens holding jig 500 shown in FIGS. 9(a) through 9(c), furthermore, the hard coating solution tends to gather on the lens near the lens edge contacted by the lower side holder 46. When the hard coating solution collects, it builds up and makes the resultant hard coating thick, developing an optical appearance defect.
As shown in FIG. 11(a), if a edged lens shape L3 is taken from a circular lens L1 and set into an eyeglass frame, then since the distance between the edge of the lens L1 and the edged lens shape L3 is sufficiently large, a solution buildup F near the lens edge does not affect the edged lens shape L3 and causes almost no problems.
If a convex lens is to be thin in its entirety, as shown in FIG. 11(b), then the lens is ground to a reduced thickness within a range which does not affect the edged lens shape L3, producing a noncircular lens L2 such as an elliptical lens. However, since the distance between the edged lens shape L3 and the edge of the noncircular lens L2 is not sufficiently large, the solution buildup F near the lens edge may be positioned in the edged lens shape L3 and cause an appearance defect, tending to lower the yield of noncircular lenses L2.
The present invention has been made under the above circumstances. It is a second object of the present invention to provide a lens holding jig which is capable of reducing, as much as possible, the generation of a solution buildup at a lower side holder for holding a lower edge of a lens.